The present invention relates to vacuum flitch table systems and, more particularly, the invention relates to vacuum valves for flitch tables that can provide a reliable vacuum seal notwithstanding the presence of debris.
In the veneer slicing industry, a flitch is carried on a flitch table for slicing. The flitch is held in place on the flitch table by a set of dogs, and, typically, the table moves the flitch in a reciprocating motion past a slicing knife which slices the veneer from the flitch. Since the dogs can extend outwardly from the flitch table as much as ⅝ inch, a substantial thickness of the flitch is unavailable for slicing. Thus, the loss of up to as much as ⅝ inch of a flitch to slicing represents a major loss of product.
Vacuum flitch tables have been proposed to overcome the problem by eliminating the need for dogs. For example, U.S. Pat. No. 3,905,408 to Hale discloses a vacuum flitch table that includes a vacuum cell plate that incorporates a plurality of vacuum cells. Each vacuum cell is equipped with a check valve to open the cell to a vacuum source. When a flitch is placed on the flitch table, the flitch seals the vacuum cell and the check valve opens to impose vacuum on the vacuum cell and hold the flitch in place.
Conventional vacuum flitch tables suffer from a serious disadvantage in that the valves tend to get clogged by debris from the flitch, such as resin, dirt and splinters.
One attempt to overcome the problem of clogged valves is disclosed in U.S. Pat. No. 5,385,184 to Mellor. The '184 patent discloses a vacuum flitch table that incorporates a ball valve to open each cell to the vacuum source and positions a screen between the flitch and the ball valve to keep the valve from clogging with debris such as wood fibers and dust, sap, and the like. However, the problem was not completely solved because the debris collection point was merely moved from the valve to the screen. The screen collects the debris and eventually clogs. When the screen gets clogged, an operator must unclog the screens, typically by actuating a blow-back system to blow the debris out of the screen by compressed air.
Another attempt to overcome the problem of clogged vacuum valves for use with vacuum flitch tables is disclosed in U.S. Pat. No. 5,590,700 to Brand. The valve of the '700 patent was directed to the resistance of clogging and the elimination of blow-back systems by cutting debris that might otherwise interfere with operation of the valve each time the valve moved between an open and a closed position.
Notwithstanding these prior developments, a need remains for a valve to reliably control the application of vacuum to a vacuum flitch table in the presence of the resin, wood fibers, dirt and other debris that is inevitably drawn from the flitch by the applied vacuum.